Well if you're tracking shields you're probably healing in which case I would recommend Grid. I'm going to assume you're at least familiar with it. The bottom right square on peoples frames is yellow when they have shield on them and a grey square when it's just weakened soul. It's not an exact number (although if I click on them I can see the seconds remaining on the shield and the debuff with ClassTimers) but I can monitor the whole raid and tell if they have a shield on them and the exact moment when I can put a new one on them with those 2 color indicators in the bottom right with Grid.
The above post mentioned something about DoT Timers, not sure if you can work with that for what you need as I don't play shadow and never used it.
For Prayer of Mending I use PoMTracker. It works awesome and I sit it right above my Grid. It shows the number of charges left and the persons name it is currently on. Underneath that it shows a shrinking bar going from right to left indicating how long it has left. When the bar is blue PoM is on CD and when the bar is red it can be cast again.
Here's a pic.
On the left you see the blue bar underneath indicating PoM is still on cooldown. The right pic shows the bar turned red indicating you can cast PoM again and is half way done. You can also see there are 6 charges left and it's currently on Glowyrm!
I know the one about grid, but this is more for PVP side.
I thought having a list on my screen to show who has my shield and the time they have remaining and same goes with my prayer mending would make it more clear to see.. but im not having much luck finding this